The present invention relates to a method for purification of the reaction mixtures arising from the production of diaryl carbonates. In particular, the present invention relates to removal of lead species from the process streams arising from the production of diaryl carbonates.
Diaryl carbonates, and diphenyl carbonate in particular, are valuable monomer precursors for the preparation of polycarbonates by melt transesterification. An advantageous route for the synthesis of diaryl carbonates is the direct carbonylation of aromatic hydroxy compounds by carbon monoxide and an oxidant in the presence of a catalyst.
A wide range of catalysts may be used in this preparation of diaryl carbonates. For example, U.S. Pat. No. 4,187,242 to Chalk discloses catalysts derived from Group VIIIB metals, i.e., metals selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or complexes thereof. U.S. Pat. No. 5,231,210 to Joyce, et al., U.S. Pat. Nos. 5,284,964 and 5,760,272 to Pressman et al., and U.S. Pat. No. 5,399,734 to King, Jr., et al. further disclose the use of co-catalysts, including metal co-catalyst species such as cobalt pentadentate complexes and complexes of cobalt with pyridines, bipyridines, terpyridines, quinolines, isoquinolines, aliphatic polyamines such as ethylenediamine, crown ethers, aromatic or aliphatic amine ethers such as cryptands, and Schiff bases, in combination with organic co-catalysts such as terpyridines and quaternary ammonium or phosphonium halides.
U.S. Pat. No. 5,498,789 to Takagi further discloses the use of a catalyst system comprising at least one lead compound soluble in a liquid phase, at least one halide selected from quaternary ammonium halides and quaternary phosphonium halides, and optionally at least one copper compound. As examples of suitable lead compounds, the publication disclosed lead oxides such as PbO, Pb.sub.3 O.sub.4, PbO.sub.2 and the like; inorganic lead salts such as Pb(NO.sub.3).sub.2, PbSO.sub.4 and the like and other lead compounds generally represented by the formula Pb(OR).sub.2, wherein R is an alkyl or aryl group, for example Pb(OCH.sub.3).sub.2, Pb(OC.sub.6 H.sub.6).sub.2 ; or wherein R is an acyl group, for example acid salts of lead such as Pb(OC(O)CH.sub.3).sub.2, Pb(OC(O)CH.sub.3).sub.4, Pb(OC(O)C.sub.2 H5).sub.2 and the like; and lead complex compounds such as phthalocyanine lead.
As can be seen from the above brief review, the crude reaction mixtures arising from the production of diaryl carbonates can contain complex mixtures of catalyst and co-catalyst metals, and organic products and by-products. The cost of commercially implementing direct oxidative carbonylation depends heavily on a combination of the efficiency of the catalyst system and on the ability to reclaim and recycle unconverted hydroxy aromatic starting material and the expensive catalyst components. Even reclamation of less expensive metals such as lead can lead to financial and environmental advantages. Accordingly, there remains a need for efficient, convenient methods for the removal of lead from the reaction mixtures generated in the carbonylation of hydroxy aromatic compounds.